AbstractThe SOS Chromotest was carried out on leachates of ten industrial wastes with the standard procedure and a miniaturized version with microplates. The two methods gave identical results in nine samples (eight negative and one positive). A simple additional manipulation is described for the identification of the false positive response that is frequently observed with complex mixtures. It involves challenging the SOS Chromotest bacteria with samples (having previously shown a positive genotoxic response) just before the enzymatic activities (i.e. β‐galactosidase and alkaline phosphatase) are estimated colorimetrically. This additional step eliminates discrepancies between the results for the standard and the miniaturized procedures.